Vehicle front fork

ABSTRACT

Herein disclosed in a front fork for a two-wheeled vehicle, in which a fork tube carrying a piston at its lower end is slidably fitted in a bottom casing filled with working oil, in which a seat tube fixed to the bottom portion of the bottom casing has its upper portion inserted into the fork tube, in which a check valve operating so as to be closed upon contraction of the fork and opened upon extension of the fork is carried by the lower end of the fork tube, in which a check valve operating so as to be opened upon the contraction of the fork and closed upon the extension of the fork is slidably fitted on the fork tube above the piston, in which as orifice is formed in the fork pipe thereby to provide communication between the inside and outside thereof, and in which a plurality of orifices are formed in the seat tube and are arranged in the axial direction of the same. The front fork tube constructed can improve the riding comfort and hold steering stability which are independent of the conditions of a road surface.

This invention relates to a front fork for a two-or three-wheeledvehicle. A construction of such fork has been disclosed in U.S. Pat. No.4,013,149 to Fabre et al. and Japanese Utility Model No. 9060/53published on Mar. 9, 1978 and is so constructed as to generate a weakdamping force upon contraction and a strong damping force uponextension. Consequently, especially when the front wheel drops in adepression in the road, damping force can be hardly attained. Even whenthe front wheel runs over a bump in the road, the shock is transmittedto the rider, because the initial damping force is relatively strong, sothat sufficiently satisfactory riding comfort and steering stabilitycannot be attained.

According to the present invention a front fork for a two wheeledvehicle comprises a bottom casing filled with working oil and slidablyreceiving a fork tube carrying a piston at its lower end, a seat tubefixed to the bottom portion of the bottom casing and extending into thefork tube, a first check valve carried by the lower end of the fork tubeand operating so as to be closed upon contraction of the fork and openedupon extension of the fork, a second check valve overlying the pistonand operating so as to be opened upon contraction of the fork and closedupon extension of the fork, the fork tube having an orifice forproviding communication between the inside and outside thereof and anaxially extending row of orifices formed in the seat tube.

The effect of such a construction can best be described with referenceto a particular example, as illustrated in the accompanying drawings, inwhich:

FIG. 1 is a longitudinal section showing a construction of front forkaccording to the present invention;

FIG. 1A is a perspective view showing a check valve used in the presentinvention;

FIGS. 2 to 4 are graphs illustrating characteristics of the front forkof FIG. 1.

As shown in FIG. 1, a piston 2 is carried on the lower end of a forktube 1 which is fastened to the front portion of a vehicle body. Thefork tube 1 is slidably fitted in a bottom casing 3 which is fastened tothe axle of the front wheel and a seat tube 4 fixed to the bottom of thebottom casing extends upwardly into the fork tube 1.

The bottom casing 3 is equipped with an oil seal 5 and a tube guide 6 atits upper end and is filled with working oil 7. The fork tube 1 isequipped at its upper end with an air valve 8 which confines air orother gas under a suitable pressure in the space above the oil surface.A suspension spring 9 is mounted under compression between the upperside of a spring seat 4' mounted on the upper end of the seat pipe 4 andthe upper end of the fork tube 1 and a balance spring 10 is mountedunder compression between the lower side of the spring seat 4' of theseat tube 4 and a valve seat 1' fitted in the lower end of the fork tubeso that the fork tube is stabilised at a position where the differencein compression between the springs 9 and 10 is balanced by a load.

At the lower end of the fork tube 1 is a check valve 12 which is formedby a ring slidably fitted on the seat tube 4 and formed with a radialgroove 12' in its lower side as shown FIG. 1A and which is urged towardsthe valve seat 1' by a spring 11. An annular check valve 13 is alsoslidably fitted on the fork tube above the piston 2 and the fork tube isformed with oil ports 14 opening into the check valve 13. The checkvalve 12 is respectively opened and closed upon the extension andcontraction of the fork, whereas the check valve 13 is respectivelyclosed and opened upon the extension and contraction of the fork. Thefork tube 1 is formed with spaced orifices 15 and the seat tube 4 isalso formed with a series of orifices 16 to 20 and so on which arespaced in the axial direction.

When the front fork having the construction thus far describedcontracts, the oil pressure in the oil chamber A below the piston 2 israised so that the check valve 12 is held in its closed state, aspreviously described, and the oil pressure in the oil chamber B in thefork tube 1 is also raised so that the check valve 13 is opened therebyto supply the oil from the oil ports 14 to the oil chamber C above thepiston 2. As a result, the oil below the piston 2 flows into the forktube 1 via the orifices 18, 19 and so on, which lie below the checkvalve 12, thereby generating a damping force corresponding to the totalarea of the aforementioned orifices. Therefore, if the descendingvelocity of the fork tube is assumed to be constant, the damping force Fis increased with the increase of the down stroke L thereof, i.e. assuccessive orifices are covered by the piston, as indicated by a curve αin FIG. 2.

On the other hand, when the contracted fork is caused to extend, the oilpressure in the oil chamber C above the piston 2 is raised to close thecheck valve 13, and the oil pressure in the oil chamber A below thepiston is lowered so that the check valve 12 is opened to supply the oilchamber A below the piston 2 with the oil in the oil chamber B in thefork pipe 1. On the contrary, the oil in the oil chamber C above thepiston 2 flows into the fork tube only via the orifices 15 so that arelatively strong damping force is generated. Specifically, if theextending velocity of the fork is assumed to be constant, the dampingforce F in this case is expressed by a straight β in FIG. 2, since theamount of the oil flowing from the oil chamber B to the oil chamber Athrough the orifices 16 to 20 can be considered to be such that itsupplements the shortage in the oil chamber A, namely, the amount ofwhich is negligible.

Thus, a front fork according to the present invention has its dampingforce augmented as the fork tube moves down, the rate of increase offorce being greater the more the fork is contracted. As a result, if thefront wheel is subjected to shocks when it passes over a bump in theroad, the fork tube easily moves down to absorb the shock with a weakdamping force during an initial stage. Moreover, since the damping forceincreases and the descending velocity is gradually reduced with thedownward movement of the tube, the absorbed energy E is substantiallyuniformly distributed over a large proportion of the down stroke of thetube, as indicated by a curve γ in FIG. 3.

A front fork according to the prior art has its damping force F heldsubstantially at a constant level independently of the tube position L,as indicated by a chain line in FIG. 2, so that the absorption energy Eduring the contraction is expressed by a chain curve in FIG. 3. As aresult, at the instant when shocks are received from the road surface,they are transmitted to the vehicle body and reduce the riding comfort.In a front fork according to the present invention, on the contrary theinitial damping force is made sufficiently weak as indicated by thecurve γ, to improve the riding comfort and the absorption energy overthe whole stroke during the contraction, i.e. the area enclosed by thecurve γ is made sufficiently larger than the area enclosed by the chaincurve according to the prior art that the shock energy can beeffectively absorbed during the contraction stroke.

As a result, the damping force upon extension is set at such a low levelas is illustrated in FIG. 2 so that shocks caused when the front wheeldrops in a depression in the road surface can be sufficiently damped. InFIG. 4 a solid curve δ expresses the relationship between thecompression stroke L and the time T for a front fork of the presentinvention, i.e. the vibration waveforms, whereas a chain curve expressessimilar waveforms of a front fork of the prior art.

Since the shock energy is absorbed during the contraction stroke by afront fork according to the present invention, the amplitude is soreduced as to improve the riding comfort. Moreover, if a braking forceis applied during the running operation, the weight of the rider isapplied to the handle bars so that the fork is contracted. However,since the damping force is strengthened by that contraction, aspreviously described, the handle can be prevented from diving to anyconsiderable extent, thereby holding the steering stability. Even if thefork is contracted almost to its limit, the damping force is also sogreatly increased to provide a beneficial effect that it is unnecessaryto additionally provide an oil lock mechanism.

I claim:
 1. A front fork for a vehicle comprising a bottom casing filledwith working oil and slidably receiving a fork tube having upper andlower ends and carrying a piston at its lower end, a seat tube havingupper and lower ends and having the lower end fixed to the bottomportion of the bottom casing and extending into the fork tube, springmeans mounted under compression extending between the upper ends of saidfork tube and said seat tube and a balance spring mounted undercompression between the upper end of the seat tube and the lower end ofthe fork tube so that the fork tube is stabilised at a position wherethe difference in the compression between the two springs is balancedwith a load, a first check valve carried by the lower end of the forktube and operating so as to be closed upon contraction of the fork andopened upon extension of the fork, a second check valve overlying thepiston and operating so as to be opened upon contraction of the fork andclosed upon extension of the fork, the fork tube having an orifice forproviding communication between the inside and outside thereof and anaxially extending row of orifices formed in the seat tube.
 2. A frontfork according to claim 1 wherein the first check valve includes a ringwith a radial groove in its lower side and a spring for urging the ringagainst a valve seat.
 3. A front fork according to claim 1 or claim 2wherein the second check valve includes an annular valve body slidablyfitted on the fork tube, and oil ports formed in the fork tube insidethe valve body.
 4. A front fork according to claim 1 wherein the forktube has its upper end portion filled with gas.